This project involves crystallographic studies of thymidylate synthase [TS], the sole de novo pathway for dTMP synthesis in most organisms, which is an essential enzyme and target for anti-cancer/anti- proliferative drug development. We are engaged in crystallographic studies of kinetically characterized L. casei TS mutant enzymes, complexed with the substrate, directed toward understanding: i) the structural basis of substrate binding/affinity and specificity, and ii) the role of ordered water molecules in the binding pocket in substrate binding/affinity and specificity. The Computer Graphics Laboratory resource is being used for the purposes of visualizing the 3-dimensional crystal structures arising from the project and the models developed in order to understand the experimental results.